This invention relates to the ventilation of the so-called pockets in the dryer section of a papermaking machine. More specifically, the invention relates to a type of roll, commonly referred to as a "pocket-ventilating roll" in the papermaking industry, which removes moist air from the dryer section and replaces it with relatively dry air. Still more particularly, the invention relates to a unique design for a pocket ventilating roll which has a plurality of separate, longitudinally extending chambers for alternately conducting moist air and dry air, and which moist air and dry air might travel in opposite directions during at least a portion of their route of travel.
In the dryer section of a papermaking machine, a large number of dryer rolls are arranged in one or more tiers and the traveling paper web to be dried is wrapped around the peripheries of the dryer rolls to be heated to drive moisture from the web. A felt or fabric is utilized to cover the paper web over its outer surface while pressing the inner surface of the web against the surface of a dryer roll to optimize the drying process.
Between the heated dryer rolls are a plurality of so-called felt, or turning, rolls over which the dryer felt, or felts, are directed to maintain the paper web in contact with the preceding dryer roll as long as possible, and to bring the web into contact with the next dryer roll. These felt turning rolls can be the pocket ventilating rolls of this invention.
The spaces between dryer rolls and the traveling web and dryer felts are known as pockets, or pocket spaces. During the operation of the dryer section in the papermaking machine, the air in the pockets in the center of the papermaking machine, that is to say inwardly from each side of the machine, becomes very moist with the water driven out of the heated paper web. The air nearest the ends of the rotating dryer rolls migrates out of the pockets and is replaced by somewhat less moist air by convection, but the moist air in the pockets in the center of the machine tends to be trapped there. This inhibits the web drying process.
The difference in the moisture content of the air in the middle of the machine transversely of the web compared with the moisture content of the air near either end of the rolls tends to produce a web at the reel having relatively dry edges with a relatively moist center area. This, of course, is deleterious to the overall quality of the paper product produced.
Prior pocket ventilating rolls attempted to alleviate this problem by directing relatively dry air inwardly longitudinally along one side of the hollow core body of the roll inwardly to be dispersed in the pocket while simultaneously urging moist air into the roll on the other side of the hollow core for removal through one end of the roll. Such a roll operates somewhat satisfactorily, but there are problems associated with the thermal bowing of the hollow, internal, stationary core body, or center shaft, of the roll due to temperature differentials caused by the differences in temperature of the air being conveyed into the roll and the air being conveyed out of the roll. These problems cause seal leakage, excessive seal wear, vibration of the roll and require special handling to equalize temperature along the length of the roll during start-up procedures.